VCAT
The invention is directed to novel hardware and software systems, methods, devices and configurations in virtual concatenated signals, including new protocols used to enable TDM (Time Division Multiplexed) networks to better accommodate data traffic.
Such systems pertain to the efficient transport of data services over Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH). The term TDM can be used to represent both SONET and SDH. New protocols, including Broadband Low Order (LO) and High-Order (HO) Virtual Concatenation (VCAT), specified in the T1.105 addenda, and G.7042 Link Capacity Adjustment Scheme (LCAS) allow TDM networks to better accommodate data traffic. The International Telecommunication Union (ITU) has published standards regarding LCAS and for virtual concatenated signals. The ITU-T recommendation G7042/Y.1305 defines the required states at the source and sink side of the network link as well as the control information exchanged between both the source and sink side of the link to enable the flexible resizing of the Virtual Concatenated signal. The actual information fields used to convey the control information through transport networks are defined in ITU-T Recommendations G.707, G783 for SDH and ITU Recommendations G.709 and G798 for OTN. All of these are universally recognized and well known and available to those skilled in the art, and are incorporated by reference for purposes of this application, including the commonly used definitions included therein.
Virtual concatenation provides the ability to transmit and receive several noncontiguous STSs/VCs fragments as a single flow. This grouping of STSs/VCs is called a virtual concatenation group (VCG). Using the same example as in the previous section, the STS-3c payload can be converted to a VCAT payload and be mapped noncontiguously to three STS-1s.
The VCAT notation for SONET is STS-n-mv, where n is the size of the noncontiguous STS fragments that will be used to transport the entire VCG. The value of m is the total number of n fragments that it takes to make up the total VCG. The “v” indicates that this is a VCAT payload. So in the preceding example the VCG would be an STS-1-3v flow.
Three STS-1s make up the total flow of an STS-3. An STS-12c can be broken down into STS-1 or STS-3c fragments; thus it can be transported as an STS-1-12v or STS-3c-4v. The VCAT notation for SDH is VCn-mv, where the definitions of n and m are the same as used for SONET. For example a VC-4-16c payload can be mapped to a VC-3-16v or VC-4-4v
Unlike noncommon concatenations, such as an STS-24c/VC-4-8c, VCAT only needs to be implemented in the path-terminating devices because resequencing and the indication of multiframing is performed via the H4 byte, a path overhead field. Path overhead is only used at the source and destination of the TDM flow. Since VCAT streams are composed of STS-1/VC-3 and STS-3c/VC-4, which are supported by virtually all SONET/SDH devices, the legacy non-path-terminating transport equipment need not support VCAT. Therefore, the utilization gains can be enjoyed by the rest of the TDM network without the need to map circuits to larger fixed concatenations and without the need to aggregate smaller flows to larger fixed concatenations, as explained earlier.
An additional feature that VCAT indirectly supports—although it does not specify the means—is the ability to provide hitless resizing of STS/VC paths. LCAS is one scheme that defines hitless resizing. Since a VCAT's payload is broken into several fragments, adding or removing bandwidth can be accomplished by adding fragments to or removing them from an existing flow, as discussed in the following section. VCAT also does not specify a protection scheme, but the LCAS scheme can also provide protection control.
Finally, additional wideband support for virtual tributary (VT)-1.5s (1.544 Mbps) and VC-12 (2.048 Mbps) is available for even smaller granularity selection. This is low-order (LO) VCAT.
LCAS
Changing a customer's bandwidth profile is always an issue. It is important to take something that works, change it, and make sure it works again without anyone noticing. Most customers demand this, and many have it written into their service contracts. The best-case scenario for adding or decreasing bandwidth occurs when there is enough bandwidth for both the old and new paths to coexist during re-provisioning. After the two circuits are up, a bridge-and-roll is performed to move the customer to the new circuit. But when there is not enough bandwidth for the two flows to coexist, the old circuit must be removed before the new circuit can be set up, resulting in a customer outage. The aim of LCAS is to make changing bandwidth a simpler and safer task.
LCAS provides a control mechanism for the “hitless” increasing or decreasing of the capacity in a VCG link to meet the bandwidth needs of the application. It also provides the capability to temporarily remove member links that have experienced a failure. The LCAS assumes that, in cases of capacity initiation, increase, or decrease, the modification of the end-to-end path of each individual VCG member is the responsibility of the network and element management systems. That is, LCAS provides a mechanism for bandwidth reprovisioning, but it is not the controlling mechanism that decides when or why the operation is made.
Features of LCAS include the ability to increase and decrease VCG capacity in increments of its fragmented bandwidths, hitless bandwidth changes, automatic removal of failed VCG fragments without removing the entire VCG, as well as dynamic replacement of failed fragments with working fragments, interworking of LCAS VCG to non-LCAS VCG; that is, a LCAS transmitter can transfer to a non-LCAS receiver and visa versa, unidirectional control of a VCG, giving the ability to have asymmetrical connections, and many other features. These features offer a list of benefits that can greatly improve transport networking. LCAS offers the flexibility to add and remove bandwidth capacity within a VCG without affecting service or taking down the VCG. This not only saves provisioning time, but it eliminates the restriction of working during the “maintenance window.” In addition, less planning is needed, because the engineer needs to find only the incremental bandwidth for the circuit, rather than the additional bandwidth required for a bridge-and-roll.
Another key benefit is that LCAS adds and deletes bandwidth in VCG increments. This allows the provider to offer a greater range of SLAs. Also adding to the SLA feature list is LCAS's ability to add bandwidth on demand. Therefore, it will aid in the creation of customer-based on-demand bandwidth changes—another advantage for the service provider.
In addition to management provisioning and customer invocations, LCAS can work in conjunction with signaling protocols to dynamically change the flow of traffic in a network. One application for this would be network-wide or span-based load balancing. Furthermore, load balancing/network recovery could be biased toward those customers that pay for a higher availability. A load-sharing restoration scheme potentially can be a component of a service that, when combined with packet-level prioritization and congestion-avoidance schemes, produces new types of enhanced service offerings.
VCAT flexibility could also be enhanced with LCAS. This will greatly improve the provider's ability to provision flexible and efficient SLAs as well as to provide dynamic TDM path restoration.
The SONET/SDH (“Synchronous Optical Network/Synchronous Digital Heirarchy”) transport hierarchy was designed to provide telecom carriers a practical means to carry voice and private line services using time-division multiplexing. In its initial design, SONET/SDH maintained a fixed hierarchical structure with a limited set of data rates (e.g. 51 Mb/s, 155 Mb/s, 622 Mb/s, 2.5 Gb/s, 10 Gb/s, 40 Gb/s). With the growth of the Internet and Enterprise data networks, and as the range and type of traffic has expanded, there is a need to make this structure more flexible and powerful.
The introduction of a set of next-generation SONET/SDH technologies, consisting of GFP, VCAT and LCAS, transforms the SONET/SDH transport network into a flexible and efficient carrier of data as well as voice circuits, while retaining the superior operations and management functionality built into SONET/SDH standards for performance monitoring and fault isolation. GFP, VCAT and LCAS have been developed in parallel and their main benefits are realized when they are used in combination. They complement each other to provide efficient utilization of transport resources and elastic bandwidth control. Components, vendor equipment and test equipment based on these are becoming mature and interwork to a high degree. Each of the three cornerstone technologies mentioned above has unique contributions to next-generation SONET/SDH:
Generic Framing Procedure—GFP (ITU-T G.7041 and G.806) is a lightweight encapsulation method for any data type providing flexible mapping of different bitstream types into a single byte-synchronous channel. It provides efficient encapsulation with fixed, but small overhead per packet. There are two main types of GFP:
Frame-based GPF (GFP-F) stores and forwards entire client frames in a single GFP frame. This is the preferred method for most packet types.
Transparent GFP (GFP-T) provides low latency by transporting block-coded signals for applications such as storage area networks, or SANs.
Virtual Concatenation, VCAT (ITU-T G.707 and G.783), is an inverse-multiplexing technique to combine arbitrary SONET/SDH channels to create a single byte-synchronous stream. Unlike continuous concatenation that needs concatenation functionality at each network element, VCAT only needs concatenation functionality at the path termination equipment. VCAT can transport payloads that do not fit efficiently in standard STS-Nc or VC SPE sizes typically supported by existing SONET/SDH NEs. VCAT functionality is only required only at Path Terminating Elements, not each NE in the path. VCAT uses smaller bandwidth containers to build a larger bandwidth end-to-end connection. The individual containers may be diversely routed, with compensation made for differential delay between each container.
Link Capacity Adjustment Scheme, LCAS (ITU-T G.7042, G.806 and G.783), is a signaling mechanism to dynamically and hitlessly adjust the size of a container transported in a SONET/SDH Network with VCAT. It is an extension to VCAT allowing dynamic changes to the number of SONET/SDH channels in use and is carried in-band on Path overhead bytes.
LCAS coordinates bandwidth adjustment on end-points, assuming the Trail has already been provisioned. It also includes optional failover recovery features.
At the edge of the SONET/SDH network, a device such as a Multi-Service Provisioning Platform (MSPP) may exist to adapt the Ethernet physical interface for transport in the SONET/SDH network. The Preamble and Start of Frame Delimiter in the MAC frame are removed and the remainder of the MAC frame (including the Source and Destination Addresses, Length/Type fields, MAC data, padding bytes and Frame Check Sequence) is mapped into the GFP payload. GFP overhead bytes are added and GFP frames are assigned to VCAT groups (VCGs), which may take diverse paths across the network. (Note that in the OIF World Interoperability Demonstration, the SONET/SDH network was comprised of multiple domains and multiple carrier labs utilizing equipment from different vendors.) LCAS signaling is intended to add or remove members of the VCG link to adjust to the bandwidth needs of the application and respond to failure or restoration of VCG member links At the network egress MSPP, the payloads from the VCGs are demapped from GFP, reassembled in time sequence, multiplexed and transmitted on an Ethernet physical interface.
Testing for service adaptation features in the OIF World Interoperability Demonstration focused on four areas:                Throughput of Ethernet Private Line services over SONET/SDH infrastructure        Accommodation of partial-rate and full-rate Ethernet transport by GFP and VCAT        Resilience of the adaptation to different network characteristics (differential delays)        In-service reaction to increased/decreased bandwidth demands and to network failure conditions with LCAS        
These test cases demonstrated not only interworking between different vendor equipment but also interworking between the essential features of GFP-F, VCAT and LCAS.
As an example of an application of VCAT and LCAS, illustrations of an Ethernet service that can be provided by a VCAT system using LCAS are shown in FIGS. 1A-1D. FIG. 1 illustrates an Ethernet system having service provided by VCAT and LCAS. The link between node A and node Z transports Ethernet frames using a virtual concatenation group of three members, and can be any number of members. The three separate LCAS protocols constantly monitor each peer location, including LCAS-a of node R talks with LCAS-a of node Z, LCAS-b(R) with LCAS-b(Z), . . . LCAS-n(R) with LCAS-n(Z), and so on. The LCAS protocol establishes the state machine and much of the configuration parameters for such a system, but it does not specify particular implementations of components for performing LCAS functions in the particular nodes. For communication, each node would need a send and receive component for transmitting and receiving data from CP1 and CP2 for example, and multiple nodes are possible.
The CP 1 sends an Ethernet signal having packets to the node R, where it is adapted to the data traffic were the Ethernet packets are processed according to a generic framing procedure (GFP). The frames are then segregated according to a VCAT process.
Historically, packet-oriented, statistically multiplex technology such as IP or Ethernet, do not match well the band with granularity provided by a contiguous concatenation. VCAT is an inverse multiplexing technique that allows granular increments of bandwidth and single VC-n units. At the source node, VCAT creates a continuous payload equivalent to X times the VC-n. The set of X containers is known as virtual container group (VCG), and each individual VC is a member of the VCG. Lower-order virtual concatenation (LO-VCAT) used X times VC 11, VC 12, or VC 2 containers (VC 11/12/2—X v X equals 1 . . . 64). Higher-order virtual concatenation (HO-VCAT) uses X times VC 3 or VC 4 containers (VC ¾—X v, X equals 1 . . . 256), providing a payload capacity of X times 48, 384 or 149, 760 kbit/s.
Referring to FIG. 1A a virtual concatenation operation, in particular, an Ethernet service provided by VCAT/LCAS is illustrated. The processor CP1 is connected to an Ethernet connection where flow control is performed according to Ethernet protocol. According to the new protocol, node R receives the Ethernet signal in a generic framing procedure for traffic adaptation followed by payload segregation according to virtual concatenation operations. Traffic control is performed according to an LCAS protocol before the node R exports the data via a cross bar switch, here shown as a legacy STH. The Ethernet frames are appended to VCAT member information as illustrated and are transported to a node Z via the legacy STH. Node Z similarly configured as node R, is configured to receive the data via the LCAS operation protocol and payload aggregation is performed to reconfigure the data for use by controller CP2 receiving Ethernet signals. The flow control is similarly operated or performed via flow control of the Ethernet protocol. For telecommunication, each node has the ability to send and receive the Ethernet signals. With virtual concatenation, the legacy STH has the ability to increase its bandwidth and efficiency with LCAS operations performing the traffic control.
Referring to FIG. 1B, virtual concatenation is illustrated graphically where the contiguous payloads VC ¾V, for example, are broken down into X segments, where each segment has multiple sequences and associated MFI numbers. Each segment has X multiples of VC 3, virtual concatenation groups, where each segment corresponds to a particular MFI number and sequence. As can be seen, the virtually concatenated groups (VCG) are transmitted individually.
Referring to FIG. 1C, a VCAT channel managed by LCAS is illustrated. Between node A and node B, signals are transferred, Tx and received, Rx, via VCGs, virtual concatenated groups in a pipeline manner. Each node has a source and a sync, and each also has corresponding LCAS configurations. The LCAS helps network operators efficiently control NG SDH connections established at VCAT sites. The use of LCAS is not compulsory, but improves VCAT management. As can be seen, the member states between the source and sync correspond with 4/idle, add/fail, norm/ok, DNU/fail, and remove/ok. As also can be seen, the transmission channels A, B, C, and D are shown as channels in a transfer mode, and the corresponding channels H, I, J, and K are illustrated as channels that transmit from the source to the sync. From node B to node A.
Referring to FIG. 1D, a K4 multi-frame (VCAT and LCAS codification), is illustrated. The lower order path overhead is shown in position to 17-20 within the K4 multi-frame. Also, in the multi-frame is the MFI number, the sequence number, control number, control bit, RS-ACK, an MST number and the CRC-3. Also, the K4 super frame is illustrated with corresponding MFI numbers, sequence numbers control and CRC-3 numbers. (SQ-sequence indicator in the VCG [0. . . ]). MFI: multi-frame count indicator [0 . . . 31]. The K4 super frame has a time length of 512ms. K4 is part of the LO-PO overhead and is repeated every 500 milliseconds. 32 bits are sent in a complete multi-frame, which takes 16 milliseconds to repeat (500×32=16 ms). The bit-2 super frame is made up of 32 multi-frames and takes 512 milliseconds to repeat. On the high order side, referring to FIG. 1E, the H4 multi-frame is illustrated in VACT and LCAS codification. H4 is part of the HO-PO (high order overhead). A4 is repeated every 125 milliseconds. 1-6 byte multi-frames takes 16 milliseconds. A complete multi-frame of 4096 bytes takes 512 ms to repeat (125×4096 =512 ms).
Many attempts at achieving a structure that is flexible and powerful have been attempted, however, the division of functions between the hardware and software to date in conventional systems has been inequitable. In particular, when certain processes are required, such as changing membership and size of a group on the fly, the operations in the process tie up the membership activity.
For example, in a sonic based system, the ITU in space G707 updated the V-4 requires virtual concatination. For example, if a 7megabit channel is required, the result desired may be 7-1 megabit channels. As a result, there is need to perform the LCAS addition. There is a need to change the size and membership of the group on the fly according to the standard requirements. In conventional systems, this process is very long, and requires very high demands on the processors. The processes that are demanding, for example, are check configuration, interrupts, commands, failures, resets, and other operations. Given the new standard requirements, systems will be burdened by real-time process requirements.
Virtual Concatenation (VCAT) enables transport pipes to be “right-sized” for various data payloads by allowing SONET/SDH channels to be multiplexed in arbitrary arrangements. VCAT breaks down data packets and maps them into the base units of TDM frames; e.g., STS-1 (51 Mb/s) for SONET, and AU4 (155 Mb/s) for SDH. This data is then grouped in multiple data flows of varying size to create larger, aggregate payloads optimally sized to match available SONET/SDH pipe capacity. VCAT is applied at the end-points of the connections, which permits each channel used to be independently transmitted through a legacy transport network. Data is typically encapsulated using GFP. VCAT (defined in ITU-T G.707), combines a number of small SDH/SONET virtual-container (VC) payloads to form a larger Virtual Concatenation Group (VCG). VCs come in three different sizes—with VC-12 providing about 2 Mbit/s, VC-3 about 50 Mbit/s and VC-4 about 150 Mbit/s—so that an 8 Mbit/s data flow, for example, would be made up of four VC-12s. Creating these finely tuned SDH/SONET pipes of variable capacity improves the scalability and efficiency of data handling while also controlling quality of service (QoS) and customer service-level agreements. The VCG is treated as a group of independent VCs, which means that each VC can exploit any available time slot across an end-to-end path and the VCG is reformed at the other end. For example, the 8 Mbit/s payload described above can be split across four VC-12s anywhere within the overall SDH/SONET signal.
Equally important for the flexibility of data transport over SDH/SONET is the Link Capacity Adjustment Scheme (LCAS), described in ITU-T G.7042, which enables the payload of the VCG to be adjusted by adding or removing individual VCs. The LCAS recommendation provides the mechanism for signaling the demand change between the two end points, without packet loss, as the payload capacity is adjusted.
Combined with Link Capacity Adjustment Scheme (LCAS, ITU-T G.7042), VCAT is a cost-effective, elastic mechanism that allows data services to be overlaid on an existing optical transport network. These standards allow a carrier to maximise revenue while using already existing and deployed technologies. This Standards-based approach of combining Ethernet and traditional voice and data services over one transport infrastructure has become increasingly popular as service providers are challenged to deliver the same (or increased) services using multiple delivery mechanisms. These requirements are particularly critical for service areas outside those traditionally covered by metro packet networks. See:    http://www.haliplex.com.au/multis1600.html
This causes a significant problem with conventional systems and related solutions. In such systems, the division of process function between hardware and software is inequitable. Since such systems were set up merely to transmit, receive and otherwise exchange merely voice data, the efficiencies required for more modern transmission of data were not realized. In network systems, quickly managing membership linkage activity, including resolving disparate membership connections, are a necessity for systems too efficiently work. For example, if a member required a 7-megabit channel, but receives seven 1-megabit channels, then LCAS addition would need to be performed in order to change the size and membership of the group. This is addressed in the ITU-T G7042. According to the new ITU standards requirement, this must be performed on the fly, or seamless. The standard, however, does not address exactly how this is to be done. This is very processor heavy, where configurations need to be checked, commands need to be interpreted, and programming must be robust in order to effectively and efficiently make the link, and to provide the VCAT required of the new standard. A simplified LCAS source and sink state machine is illustrated in FIG. 1F.
Therefore, there exists a need in the art for a system and method of performing LCAS operations in the context of a SONET based system that provides a more efficient balance between hardware and software operations in network communications, and that will be able to perform LCAS operations in a manner that is not burdened by real-time processing requirements. As will be seen, the invention provides such a system and method in an elegant manner